The significant development of these esters for the preparation of tin stabilizers of chlorovinyl polymers makes their continuous production and the improvement of the synthesis yield highly desirable. On the other hand, the quality of the stabilizers largely depends on that of the esters, in particular their purity and their stability on storage.
Although it appears easy to esterify an alcohol forming an azeotrope with water, such as 2-ethylhexanol or mixtures of isomers of C.sub.8 alcohols (isooctanol), using an organic acid stable to heat, it is no longer the same for a mercaptocarboxylic acid which is unstable to heat, such as thioglycolic acid. In fact, by dehydration, this acid forms an esterifiable oligomer leading to the production of undesirable by-products in accordance with the equation: ##STR1## wherein n is predominantly equal to 1 (dimer) but can also be more than 1. It is also known that this straight-chain dimer can easily esterify in accordance with the equation: ##STR2##
Thus, the esterification of a mercaptocarboxylic acid such as thioglycolic acid involves several competitive reactions which, depending on the operating conditions used, can cause more or less significant formation of undesirable by-products. In discontinuous operation, preventing the formation of esterified dimers and, at the same time, obtaining a high conversion of mercaptocarboxylic acid is not successful.
It has now been found that the selectivity for the desired ester can be substantially improved if the esterification is carried out continuously, regularly removing the water formed in such a way that the residual water concentration in the reaction mixture is below 0.5% by weight. It has also been found, surprisingly, that, under these conditions, the reaction time is clearly shorter than that of the discontinuous process.